In prior methods of leaching bedded ore deposits, such as uranium rollfront deposits, leachant injection and recovery wells are constructed. Leachant is introduced in the mineralized zone at the injection well. It flows through permeable rock to a recovery well in response to the gradient in hydrostatic pressure created by well recharge and discharge. Permeability [L/T.sup.2 ] is the capability, per unit thickness, of a porous rock material to convey ground water (or leach solution). Transmissivity [L /T] is permeability multiplied by thickness, and is a measure of aquifer capability to transmit ground water.
Injection and recovery well spacings of 50 to 100 feet are typical. Injection and recovery wlls are open to fluid flow only in the mineralized zone. Above and below the mineralized zone, the well is cased and cemented to prevent excursions of the leach solution from the well bore into other aquifers.
Conventional leaching operations rely on the presence of overlying and underlying rock layers having permeabilities much lower than that of the mineralized zone to confine the leach solution, from above and below. Clay or shale beds are examples of natural barriers which confine the flow of leachant within the mineralized zone.
While natural confining beds are almost always present, often they do not lie adjacent to the mineralized zone. When mineralization occurs as a narrow band of precipitate within a thicker sandstone unit, the adjacent barren sandstone generally has a higher pemeability than the mineralized zone. In this case, despite the fact that injection and recovery wells are open only to the mineralized zone the leach solution will flow preferentially through the higher-permeability barren layers, above an below the mineralized zone.
In conditions such as this, contact between the leach solution and the mineralized zone is significantly reduced and the geochemical processes of leaching are substantially inhibited.
Computer simulations of leaching hydrology and geochemistry provide a graphical means of confirming that a substantial amount of leachant does flow outside the mineralized zone in the conventional leachant flow pattern.
For example, the plot of FIG. 2 shows a cross-sectional representation of a conventional leachant flow pattern between a single pair of injection and recovery wells. The flow pattern (represented by 7 symmetric pairs of streamlines) is developed for a layered aquifer setting where the low permeability confining beds (k.sub.2) are separated from the mineralized zone (k.sub.0) by a higher permeability barren zone (k.sub.1). This corresponds to the type A or type C deposit in FIG. 1. 1. This flow pattern was generated using hydrogeologic field data from an operational uranium leach site. Streamlines in this and all subsequent plots are constructed so that an equal volume of leach solution flows between each adjacent pair of streamlines.
Field data used in this simulation is summarized as follows:
TABLE 1 ______________________________________ Hydrologic Data, South Texas Uranium Leach Site ______________________________________ Permeability Thickness Transmissivity Layer (gal/day/ft.sup.2) (ft) (gal/day/ft) ______________________________________ zone k.sub.2 .002 NON/Applicable 0 zone k.sub.1 63.5 15 952.5 zone k.sub.0 6.05 3 18.15 zone k.sub.1 63.5 10.5 666.75 zone k.sub.2 .002 NON/Applicable 0 ______________________________________ depth to zone k.sub.0 200 ft depth to water (ambient level) 15 ft well spacing 37.5 ft leachant injection and recovery rate 2 gallons/minute average leachant velocity 18.0 ft/day ______________________________________
As table 1 indicates, the ratio of permeabilities between barren and mineralized zones is k.sub.1 /k.sub.0 =10.
In FIG. 2, despite the fact that the wells are open only to the ore zone, leachant contact with the ore material is limited to a small area around the open interval of each well. Between the wells, the leach solution flows almost entirely through barren rock. This is a direct result of the greater permeability and thickness (transmissivity) of the barren zones.
When the barren rock layer intervening between the mineralized zone and the confining bed is thin relative to the mineralized zone, or has a roughly equivalent permeability, there is less migration of leach solution into the barren zone.
The streamline pattern in FIG. 3 results when the permeabilities of barren and mineralized zones are the same, i.e. k.sub.1 /k.sub.0 =1.0. In this case, approximately 21 percent of the leach solution remains entirely within the low permeability mineralized zone, 79 percent of the leach solution is wasted because its flow path is mostly in the barren zone.
In homogeneous cases like FIG. 3, it is practical to increase the injection and recovery rates of leach solution in order to increase the concentration of leach solution inside the mineralized zone. Since the injection and recovery rates in these simulations are already at their maximum practical value, given a 3 foot open interval of well casing, it is necessary to lengthen this open interval in order to increase the rates still further.
FIG. 4 is a flow simulation developed for the case where the open interval is centered on the mineralized zone, and is twice its thickness. (Only the streamlines beginning inside the mineralized zone are plotted) In this simulation 29 percent of the leach solution injected inside the mineralized zone remains inside, and this represents an 8 percent increase over that of FIG. 3. The injection and recovery rates are increased by a factor of 2 over that of FIG. 3, however.
The problem of maintaining solution/mineral contact becomes much greater if the permeability of the barren rock layer greatly exceeds that of the mineralized layer. Permeability ratios of 10:1, 100:1 and 1000:1 have been encountered at leaching operations, and in these cases, increasing the open interval is not practical because it would mean increasing leachant injection and recovery rates by a factor of 20, 200 or 2000 in order to achieve the same 8 percent increase in leachant/mineral contact observed in FIG. 4.
Therefore, it can be clearly seen that increasing the open interval in the casing, and thereby increasing the injection and recovery rates, is not a practical means of inducing greater leachant/mineral contact in situations where natural confining beds are absent.